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Network mask in IPv6 for DNS server is rejected by Netplan ("Error in network definition: malformed address")

On a recent server install the netplan apply command fails with the following error message:

/etc/netplan/0-base.yaml:18:9: Error in network definition: malformed address '2001:41d0:3:163::1/128', must be X.X.X.X or X:X:X:X:X:X:X:X
        - 127.0.0.1

Here is the (redacted) content of generated /etc/netplan/0-base.yaml:

---
network:
  version: 2
  renderer: networkd
  ethernets:
    eno1:
      addresses:
      - 2001:db8:1234:5678:9::abcd:ef/64
      - 213.186.33.6/24
      dhcp4: false
      dhcp6: false
      gateway4: 213.186.33.254
      gateway6: 2001:db8:1234:56ff:ff:ff:ff:ff
      nameservers:
        search:
        - ovh.net
        addresses:
        - 127.0.0.1
        - 213.186.33.99
        - 2001:41d0:3:163::1/128
      routes:
      - to: 2001:db8:1234:56ff:ff:ff:ff:ff/128
        scope: link
...

The 2001:41d0:3:163::1/128 IPv6 address in network.ethernets.eno1.nameservers.addresses has a network mask, which is not accepted by Netplan.

Removing the /128 part fixes the problem.